[GCC]: Pass extra flags to gcc build.

There is no way to pass extra flags to gcc though
riscv-gnu-toolchain Makefile.

This is neccesary for me, because I has to rewrite gmp,
isl, mpfr, mpc folders in source of gcc project. Moreover,
I don't want to rebuild them with gcc.

Also I think, the opportunity to pass extra flags will be
convenient for many development purposes.